Most people phrase this in an emotional context, but the meaning is ultimately the same. Personally, I find a simple mathematical ratio easier to convey than the amount of prevarication it would take to express such a thing emotionally.

When that ratio is consistently greater than 1:1, that’s a person who is worth “keeping”. If it’s only greater than 1:1 in some situations, then those are the only situations when it’s worth interacting with that person. When that ratio is consistently less than 1:1, it’s time to let that person go, and avoid him/her as necessary.

Naturally, foresight and personal preference comes into play, here. If a person is mostly troublesome, right now, but you foresee him/her being beneficial in the long term, then it might be worth keeping them around. If you’re OK with 1:1, even if it’s never greater than that, then that’s your threshold for deciding whether it’s “worth it”. Most people require a ratio much greater than 1:1 to consider it “worth it”. People with large circles of close friends that they consistently have problems with are less picky (requiring a lower ratio to be satisfied); whereas those who only really want to hang out with a few people who are particularly valuable to them are more picky (requiring a higher ratio to be satisfied). I’ve noticed that this level of “pickiness” directly corresponds with the amount of energy a person has for social interaction. Those who are more concerned with other things tend not to have any interest in those with less than, say, a 2:1 ratio of benefit:cost or happiness:trouble.

If you’re not providing at least a 1:1 ratio for someone, you’re doing it wrong. If you really want someone in your life, you need to provide them a higher ratio, and be sure that they’re doing the same for you, before committing to anything long-term.

Charity is an exception to this rule. (I’m using “charity” to refer to selfless love, rather than “giving money”, which, as an exclusive term, is a perversion of the original concept.) Charity is when someone offers you less than you would otherwise accept as a ratio of happiness:trouble, but you give that person your time, energy, and resources, anyway. You self-sacrifice for that person out of kindness. We can only do this to the extent that we have personal resources (time, energy, patience, emotional stability, money, etc.) to spare, and when we run low on this excess, we can no longer afford to give without receiving; otherwise, our own lives will suffer quite substantially. One only allows that for those we love most, such as family members. We give what we can, when we can, because we choose to; “obligation” is anemic to true charity, unless it’s someone we’re truly responsible for taking care of (such as an aging parent, a sibling in distress, or a child). Nevertheless, charity is what makes society worth having. We care for people who can’t give back as much as we give them, and, in turn, people do the same for us when we’re in need. Sadly, our society isn’t quite at the point when we can do this for each other very effectively (due to economics, and anger, mainly); but as we improve our way government and interpersonal interaction, this will slowly change–as it has been since the dawn of civilization.